Clarification relates to reducing solids content in water or other liquid streams which cannot be efficiently removed by mechanical filtration methods. Often a clarification process is used to remove non-dissolved solids before further waste processing, or may be applied to provide water which is clean enough to recycle into the same process even if not clean enough for discharge. Dissolved aeration flotation (DAF) is a widely used method to remove organic contaminants from wastewater streams such as from food processing plants. The basic process consists of injecting water saturated with gas—either air or another gas selected to be less reactive with a particular waste chemistry—into a flotation tank (“aeration”), where the gas comes out of solution forming bubbles which float to the surface of the tank. The aerated water is created by dissolving gas into the water in a high pressure environment until it reaches saturation level at that high pressure. When the gas-saturated water at high pressure is depressurized the gas comes out of solution. Bubble size and density can be controlled by varying, among other things, the maximum saturation pressure and the rate of depressurization. The rising gas bubbles adhere to particulates in the wastewater and lift them to the surface where they are skimmed off. The floating particulate matter is referred to as “retentate”, and after removal is referred to as “sludge”. Aeration may be accomplished using pressurized saturation tanks or pumps designed for the purpose, such as aeration turbine pumps.
Flocculate agents may be mixed with the wastewater prior to aeration to react with or bind to particulates, creating larger and less dense suspended coagulated particles which are more susceptible to binding with gas bubbles and thereby more effectively driven to the surface for removal. Many flocculating chemicals are known and selected based on the anticipated chemistry of the waste stream and the expected downstream uses of the clarified effluent and retentate sludge. Examples of known coagulants and/or flocculants include cationic polymers, phosphoric acid, lime and anionic polymers. The effectiveness of the system—i.e. the amount of waste removed from the wastewater stream—depends on the size and density of the gas bubbles and the amount of time the wastewater is exposed to the gas bubbles, sometimes referred to as “dwell time”. Retaining wastewater in the flotation tank exposed to aerated water for a longer period provides greater removal effectiveness. Longer dwell times are achieved by increasing the size of the tank relative to the anticipated peak flow. However, when the wastewater stream is low, such as when a plant with multiple production lines is running only a few lines, the larger DAF system will operate at low efficiency.
A persistent problem encountered with DAF systems is that they are not efficient when run at low capacity. A system designed to handle a large peak flow requires a large flotation tank volume in order an optimum dwell time, with a consequently large foot print, large pump capacities, and high expense. In order to run efficiently a low production volumes an intermediate collection system is required to accumulate wastewater and run batches through the DAF system at optimum flow rates. Intermediate collection systems consume valuable production area, cost money to build and install, and add maintenance costs.
Greater treatment capacity requires a longer and higher volume separation tank. However, this imposes minimum flow requirements on the system as a whole because the turbine pump/air injection system requires a minimum flow of either recycled clear water or clean freshwater (but adding fresh water increases the overall volume of processed liquid and essentially defeats much of the purpose of the system, which is to reduce fresh water usage and reduce waste stream volumes). Use of side-by-side systems on the same skid permits ½ to be shut down during low volume waste flow periods, such as when part of the production lines have been shutdown, but still retains full capacity during peak times. The system shares components and provides a compact, energy efficient footprint as well. The system may receive wastewater flows from a single source and split them, or may receive flows from separate sources but utilize certain common components, and still be available for a peak flow from a single source.
Additional problems arise with conventional effluent weir designs, which generally comprise a round pipe with perforations distributed along its surface, including the top surface. This design allows particulates to enter the weir pipe and foul the pipe, reducing flow and potentially contaminating the effluent discharge. This design also creates a problem of sediments accumulating on the upper surface of the weir pipe, which periodically dislodge and create spikes of particulates in the effluent. Maintenance requirements are substantially increased due to more frequent flushing required and more difficult cleaning during shutdowns.
Thus, there is a need for DAF treatment system that: (1) is compact; (2) modular to permit scaled or split operations; (3) provides improved methods for removing effluent; (4) reduces buildup of sediments on surfaces; (5) provides improved solids removal efficiency; (6) maximizes removal of effluent from sludge; (7) improves laminar flow within the separation vessel; (8) reduces water velocity within the separation vessel; (9) improves dwell time within the separation vessel; (10) provides for adjustable height risers to control system liquid level; and, (11) improves overall efficiency and cost effectiveness.